A large variety of hardness testers for measuring the hardness of solid materials are known in the art. The typical hardness tester comprises some type of indenter (which may be pointed or rounded depending on the particular intended application), a means of applying a driving force to the indenter, and a means for measuring the amount of penetration of the material being tested. Typical of such testers are U.S. Pat. Nos. 2,544,205; 2,663,185; and 3,182,491.
Somewhat special problems are presented by softer, elastic materials; and, in general, a hardness tester adapted to measuring the hardness of metals, ceramics and the like will not perform well with plastics. Several techniques, however, have been developed for testing the hardness of plastic-like materials as shown in U.S. Pat. Nos. 2,373,662; 3,406,566; 3,597,965; and 2,892,342. All of these references deal with testing the hardness of bulk materials and not with thin plastic films and the special requirements thereof.
Polymer plastic coatings are presently employed in a seemingly endless variety of protective and decorative applications. Although plastics have demonstrated a durability unparalleled by conventional wood and metal products, they are subject to gradual degradation under the influence of time, normal weathering, heat, cold, stress and strain, exposure to certain chemicals, ultraviolet light and other forms of radiation energy, and perhaps many other factors that have yet to be discovered. It will be appreciated that the rate at which such degradation occurs also varies according to the chemical structure of the plastic. The visible effects of such degradation include discoloration, warping, cracking and similar signs of brittleness, but only in the advanced stages of the degradation. Another common type of plastic deterioration is known as hydrolytic stability failure. Hydrolytic stability failure (sometimes called reversion) refers to the quality of a material which, having cured to a solid state from its original liquid state, can revert to the liquid state or soften considerably. There is a prescribed MIL SPEC for hydrolytic stability tests but it does not include coatings and thin film materials. Under particularly severe operating conditions, for example in protective applications in the aerospace industry, hydrolytic stability failure may occur long before the first patently visible signs of degradation of the plastic.
One of the earliest detectable signs of plastic degradation is a change in the hardness. Thus, a simple and accurate means for measuring the hardness of plastic films can be used not only experimentally to test the hydrolytic stability of different plastics under different environmental conditions bu to monitor plastic coatings actually in use thereby to predict possible plastic failure before it occurs.
The major difficulties in testing the hardness of plastic films all relate to the fact that indentation must be extremely small in order not to penetrate the film. For a typical film of about 5/1000ths of an inch, the typical indentation should not be greater than 5/1000th of an inch, that is, penetration of the film to the substrate beneath must not occur. Thus, factors such as friction and small amounts of angular deflection of the indenter which might go unnoticed in testing a bulk material using indentations fifty to a hundred times as great, become a major source of error in measuring indentations on the order of 1/1000th of an inch.
Another important factor in measuring the hardness of plastics is "elastic recovery" -- that is, the tendency for a material deformed under application of a force to rebound toward its original shape when the force is removed. Plastics demonstrate a much higher degree of "elastic recovery" than do metals and ceramics. Again, the factor of elastic recovery plays a much larger role in thin film testing than in testing bulk plastics. Thus, conventional methods of measuring indentations such as removing the indenter and inspecting the size of the indentation with a microscope or other optical device cannot be used for testing thin film plastics.